The present invention relates to a compensating method and device for instrumental error in rotary positive displacement flowmeters. The method and device of the invention are intended for compensation of errors due to flowmeter characteristics, correlations of temperatures and subject fluid volumetric expansion, and correlations of temperatures and viscosities of the fluid. Elliptical gear flowmeters are outstandingly superior in measurement functions to other conventional flowmeters, and therefore most suitable for further enhancing the measurement functions of the present invention in error compensation. Moreover, reliabilities of flowmeters are enhanced by the present invention.
In a rotary positive displacement flowmeter containing in the casing of a pump unit two of the same type elliptical gears having long and short diameters respectively, or "non-circular rotor" gears which are meshing mutually, leakage is prevented by limiting clearance to the minimum possible between the adjacent internal wall surface of the casing of the unit and the top of the tooth on the long diameter of the rotor when the fluid is fed at the entrance of the unit and then is passed through within the casing to be discharged at the exit, and the meshing teeth of the two rotors prevent fluid leakage with the supply and discharge side rotor teeth appearing in the direction of rotor rotation. A small leakage through the above-mentioned clearances cannot, however, be perfectly prevented. Ratios of such leakages vary with the instrument sizes, constructions, and rotational speed of rotors.
Fluids generally used in plant facilities have mostly low viscosities. Fluids of low viscosities have large viscosity variations with temperature, and therefore cannot be exempt from errors even with high precision instruments. Since in low viscosity fluid viscosity variations can result with a smallest temperature change, and therefore errors in accuracies will be inevitably produced, a high technology is needed in temperature control when a measurement is to be conducted in such cases. Generally, it is accepted that non-circular gear flowmeter have high reliabilities. The relation between the flow rate and instrumental error will be unchanged, and therefore super precision measurement can be made possible by compensation methods using microcomputers. Because of mechanical structures in rotary positive displacement flowmeters, leakage of fluids due to pressure differences cannot be perfectly prevented through even the smallest clearances between rotors or the inside wall and rotors. This leakage rate varies with viscosity changes caused by temperature changes in subject fluids and apparatus error increases or decreases throughout the flow rate range.
The action of temperature compensation mechanisms for conventional positive displacement flowmeters is to make a real-time determination of the fluid expansion and shrinkage due to temperature rise and fall during the measurement for continuous conversion to volumes at certain conditions; however the flowmeters do not indicate real correct flows because the fluid viscosity varies with the change in the fluid temperature.
In order to obtain a real correct flow, therefore, it is needed to exclude the influences on instrumental errors caused both by volumetric changes and viscosity changes due to changes in the fluid temperature. Viscosity changes in the subject fluid due to process temperatures changes, similar to specific gravities, represent correlations between instrument errors and viscosities. Consequently viscosity changes as well as specific gravity changes should be determinated on real-time to be used for compensation in conversion for a more correct flow rate. Though it is difficult to determine the smallest viscosity changes instantaneously, a precise measurement of changing temperatures on real-time is practical, and it is feasible to obtain compensation value of higher resolution by using temperature measurements.